Telomeres comprise DNA sequences at the ends of chromosomes which shorten by about 30-100 bp per year due to oxidation and incomplete DNA replication during S phase of the cell cycle. When telomeres become critically short, chromosomes form chromosome-chromosome fusions which lead to cancer, and are recognized as double-stranded breaks that activate DNA damage repair responses that lead to cell death or senescence and consequent tissue and organ dysfunction. The enzyme telomerase extends telomeres, and the limiting component of telomerase in most cells is telomerase reverse transcriptase (TERT). Short telomere length has been linked to many diseases, including, in our labs, Duchenne muscular dystrophy (DMD) and vascular disease, which includes atherosclerosis, vascular dementia, and heart disease. We (the Blau lab) found the first evidence implicating cellular senescence in DMD in 1983, and very convincing evidence linking short telomeres to DMD when we recently showed that mice with mutant dystrophin and shortened telomeres (our mdx/mTR mice) faithfully recapitulate human DMD, unlike mice with mutant dystrophin alone (Cell, 2010). Further we showed that short telomeres lead to muscle stem cell (MuSC) replicative exhaustion, and consequent inability to repair damage caused by mutant dystrophin. Indeed, DMD patients have short muscle telomeres. Thus there is a need for a safe, reliable method to extend telomeres in humans. However, all existing human- compatible methods are sporadic and slow because TERT is extensively regulated at many levels, making telomere extension through endogenous TERT highly dependent on cell type, cell cycle stage, and extrinsic conditions. Although previous work (Cooke lab) revealed that increasing telomerase activity can avert senescence in human cardiovascular cells, these studies required retroviral technology, which is suboptimal clinically, and required chronic treatment because they did not address endogenous TERT inhibition. We propose to overcome these limitations with two high-impact and broadly-applicable tools: a transient therapeutic designed to overcome TERT regulation to extend telomeres safely, rapidly, and reliably, and a delivery vehicle that will allow our TERT therapeutic to be delivered to muscle stem cells to treat DMD, and cells in other tissues, via i.v. injection. We will demonstrate these tools in cells from human DMD patients using our mouse model of DMD, the first model to faithfully recapitulate DMD, including its lethality. Our laboratories have a long history of developing innovative technologies of broad applicability. The proposed studies will enable rapid telomere-extension in vitro and in vivo in human cells, and the resulting tools will be useful in helping to prevent, dely, or treat the many major diseases in which short telomere length is implicated.